Energy is the basis of human activity. Humans are facing the exhaustion of fossil fuels, and the environmental pollution caused by fossil fuels is serious. Nowadays, almost all industrial countries are facing the crisis of energy supply.
Renewable energy is a kind of clean energy, meaning the energy that can be continuously renewed and sustainably used in the nature, in which biodiesel and fuel ethanol are striking. Biodiesel is a mixed liquid fuel of various monoesters of fatty acids obtained from animal or plant grease and short chain alcohols via transesterification, and can be used directly in an internal-combustion engine. Fuel ethanol is a high-octane fuel with the property of clean combustion and can be produced by renewable energy. However, the production of biofuels in large scale may require a large area of lands. Furthermore, the expansion of biofuels production such as ethanol production will also affect the price of grains. Therefore, the development of new energy is an urgent requirement.
Polyhydroxyalkanoates (PHA) are a kind of energy and carbon source storage materials accumulated by microorganisms under circumstances where the growth of microorganisms is unbalanced (Doi & Steinbüchel, 2002). The monomers forming PHA are various. Until now, more than 100 monomers have been discovered (Doi & Steinbüchel, 2002). 3-hydroxybutyric acid (3HB) is the most common monomer to form PHA. Typically, PHA can be represented by the following formula:
wherein n=1, 2, 3 or 4; typically n=1, i.e., poly-3-hydroxyalkanoate. m represents polymerization degree, which determines the molecular weight. R is a variable group, which can be saturated or unsaturated alkyl with a straight chain or branched chain and substituents.
When “R—” group is a substituent with less than 3 carbon atoms (that is, CH3— or CH3CH2—), PHA is called Short Chain Length PHA (abbreviated as scl PHA). In particular, when “R—” group is CH3—, PHA is called poly-3-hydroxybutyrate (abbreviated as PHB). When “R—” group is CH3CH2—, this PHA is called poly-3-hydroxyvalerate (abbreviated as PHV). 3-hydroxybutyric acid and 3-hydroxyvaleric acid can be polymerized to form poly-3-hydroxybutyrate-3-hydroxyvalerate (abbreviated as PHBV). The common examples of short chain length PHAs are PHB and PHBV. When “R—” group is a substituent comprising 3 or more carbon atoms, it is called Medium or Long Chain Length PHA.
The ester bonds in PHA can be broken to generate monomers under alcoholysis catalyzed by sulfuric acid. However, when methanol or ethanol is added during the alcoholysis, carboxyls (—COOH) in hydroxyalkanoic acid (HA) monomers generated from the degradation of PHA can react with the hydroxyls (—OH) in methanol or ethanol to generate corresponding methyl 3-hydroxyalkanoate or ethyl 3-hydroxyalkanoate (e.g. methyl 3-hydroxybutyrate or ethyl 3-hydroxybutyrate; methyl medium chain length hydroxyalkanoate or ethyl medium chain length hydroxyalkanoate).